Commonly, waste water is treated to remove contaminants that would otherwise prevent reuse of the water in potable water systems or that would contaminate receiving waters. Similarly, other waste liquids are treated to remove contaminants that might prevent reuse of the liquids in industrial processes. Such contaminants can include solid particulate matter, or dissolved acids, metallic salts and organic alcohols, as well as dissolved odorous gases. In the case of solid particulate contaminants, the contaminants are separated out of the liquid as a sludge.
Primary treatment of waste water often involves discharging the waste water into a primary settling tank. The water containing suspended solids is then allowed to remain in the settling tank for a period of time (detention time) sufficient to permit the solid particulates to gravitationally settle to the bottom of the tank as a sludge. This sludge is periodically or continuously removed from the bottom of the tank.
In another type of treatment (i.e., air flotation thickening), air bubbles are injected into the waste water prior to its admission into a subsequent settling tank so that the air bubbles can attach to the solid particulates to provide a lighter particulate condition. In the settling tank, the lighter particulates collect on the liquid surface, where they are skimmed off by an overhead travelling belt-type collector as a thickened sludge. The heavier particulates settle to the bottom of the tank as a second separate sludge.
Sludges can also be produced in a secondary treatment process as a by-product of an aerobic aeration process, wherein an aeration unit is included in aeration basins to bring gaseous air (oxygen) into contact with the particulates. Waste water flows into the aeration basins where it remains for a specified detention time. The air and incoming microorganisms are consumed by microorganisms in the mixed liquor in the aeration basins to produce a microbial floc, i.e., activated sludge. The mixed liquor from the aeration basins then flows to secondary settling tanks. The floc settles to the bottom of the secondary settling tanks, where it is periodically or continuously removed from the effluent liquid phase.
Another process involves anaerobic digestion, wherein the waste water is placed in a closed tank. The closed system allows anaerobic biochemical processes to take place, whereby some of the acids in the waste water are converted to methane gas and carbon dioxide, and solid particulates are thus consumed, or changed, to a liquid and gaseous form. A quantity of sludge remains as a by-product of the process. Typically, the tank is heated to an elevated temperature, at or near 95.degree. F., in order to keep the process ongoing. The methane gas is removed through a vent in the top of the tank, and the sludge is removed through a valved port in the tank wall.
Sludge generated as a result of any of the aforementioned waste water treatments is commonly disposed of by incineration, landfill operations, or ocean deposition. In some cases, the sludge is converted into fertilizer for agricultural use. A common problem associated with economical disposal of the various waste water sludges is the high water content of the sludge removed from the settling tanks. Typically, the sludge will have a water content of from about ninety percent to about ninety five percent, with only five to ten percent of the sludge mass being solids. This fact is an obstacle to the economical disposal of the sludge, since it increases the total sludge quantity that has to be handled. The high water content also makes the sludge somewhat more difficult to handle, because of its larger volume.
In an effort to increase the solids content in the sludge, the raw separated sludge has been subjected to various dewatering treatments, including vacuum filtration, centrifugations, pressure filtration, and drying on sand beds. However, these post-formation dewatering treatments have only been partially successful. The sludges resulting from such dewatering treatments still have only relatively small solids contents Typically, the solids content of so-called dewatered sludge is only about twenty to forty percent, with the remaining eighty percent to sixty percent being water.
The high water content of so-called dewatered sludge is due to the fact that much of the water is contained within the pores of the solid particles, or within narrow channels formed between agglomerated particles. Each particle is a relatively porous cellular structure that acts somewhat like a sponge to absorb and retain water molecules within the particle mass. Conventional dewatering processes are relatively ineffective in drawing, or extracting, water from the pores formed within such solid particles.